Instructions

Melt
butter in a skillet over medium heat. Add onions. Cook until
translucent and some of them are just barely beginning to turn golden,
stirring as needed. Remove from heat and set aside.

In a large bowl, beat the cream cheese and mayonnaise with an electric hand mixer until well blended and smooth.

Add the mozzarella, Parmesan and onions; mix until well blended.

Pour
into an appropriate size baking dish and bake at 425 degrees for 15
minutes. I have used dishes of various sizes and shapes. This dip bakes
best when it is about 3/4-inch to 1-inch deep in the dish. This may
sound really picky or technical, but per my preference, I think that
this gives the best results.

Note: This is a
mild dip, with the flavor profile focusing on the sweetness of the
onions. If you want this dip to have a 'kick', then I would suggest any
of the following additions---1 (4 ounce) can green chiles; as much
of one minced habanero as you can stand; 1/2-1 teaspoon chipotle chile
powder; minced jalapenos with or without seeds; a few dashes of your
favorite hot sauce (I like Cholula® brand chipotle flavor).

Additional note:
If you are serving this dip at a dinner party, I would suggest using
the recipe as written. If, however, you plan to use it at something like
a football game get together, then spice it up. You want the flavors to
match the event.